Chip mounting method

ABSTRACT

A method for mounting a chip on a substrate includes applying the underfill agent onto at least one of the substrate and the chip and moving the chip to the substrate to bring the bump into contact with the electrode. The method also includes steps to distribute the underfill agent in a space between the chip and the substrate, to around the bump and the electrode, heating the bump or electrode in the state that the bump is buried in the underfill agent to melt the bump or electrode so as to weld the bump to the electrode.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for mounting a chip, andspecifically to a chip mounting method for mounting a chip directly ontoa substrate in a form of face down and performing underfill between thechip and the substrate.

BACKGROUND ART OF THE INVENTION

A chip mounting method is well known, wherein a bump is formed on a chipby a solder and the like, the chip is approached to a substrate in aform of face down, and after the bump is brought into contact with anelectrode of the substrate, the bump of the chip is heated and molten tobe bonded to the electrode of the substrate. An underfill agent, whichis composed of a nonconductive adhesive, is injected into a gap betweenthe chip and the substrate in order to increase the total bondingstrength between the chip and the substrate. Further, the reliability ofthe electrical insulation between the bonded portions is increased by acondition where the underfill agent is filled around the bump of thechip and the electrode of the substrate bonded to each other.

In a conventional chip mounting method for forming a solder bump,firstly a flux is applied to the bump or a substrate side, and after thechip is mounted on the substrate at a predetermined position, the solderbump is molten by heating (in most cases, associated with pressing) andthe bump is preliminarily bonded to the electrode of the substrate.After being cooled, the flux is removed by cleaning, and thereafter, anunderfill agent is injected into a minute gap between the chip and thesubstrate from the side direction. After the underfill agent isinjected, heating is performed again, the underfill agent is re-flownand thereafter cured.

In such a conventional chip mounting method, however, when the solderbump is heated and molten in the preliminary bonding process of the chipand the substrate, there is a fear that the bonded portion issecondarily oxidized by the surrounding atmosphere. If such a secondaryoxidation occurs, it may decrease the reliability of the electricalbonding between the bump and the electrode of the substrate. In order toprevent the secondary oxidation, there is a method for purging theatmosphere around the bump using nitrogen gas and the like when the bumpis bonded with the electrode. However, such purging by nitrogen gas andthe like causes a chip mounting apparatus to become large-sized, themounting apparatus and the mounting process to become complicated, andthe cost thereof to increase, and because it is necessary to take a timefor the purging by nitrogen gas and the like, a high-speed mounting maybe obstructed.

Further, in the above-described conventional mounting method, because itis necessary to apply a flux and remove the flux by cleaning after thebonding for melt-bonding of the solder bump, there is a problem that thenumber of processes until completion of main bonding is fairly large.Moreover, the perfect cleaning of the flux is difficult, and therefore,the residual components may reduce the reliability of the bonding. Inorder to achieve a fluxless bonding, proposed is a method for using analcoholic organic material having a melting point higher than that of asolder instead of a conventional flux, and removing it by vaporizingwithout cleaning it after the preliminary bonding (JP-A-8-293665).However, in this method, basically only the cleaning process of the fluxbecomes unnecessary, and the method is poor in effect for reducing thenumber of the processes in the bonding. In particular, the processes forinjecting an underfill agent after the preliminary bonding, forre-flowing the underfill agent and for curing the underfill agent stillremain as they are. Further, in this method, as the bump pitch has beenin a fine pitch condition, it has become difficult to inject theunderfill agent.

DISCLOSURE OF THE INVENTION

Accordingly, an object of the present invention is to provide a chipmounting method which can prevent the secondary oxidation of a bump tobe heated and molten without performing the purging by nitrogen gas andthe like, and which enables to reach a main bonding process at a timewith a fluxless condition, thereby simplifying both the mountingapparatus and process greatly.

To accomplish the above object, a method according to the presentinvention for mounting a chip by bonding a bump formed on at least oneof the chip and a substrate to an electrode formed on the other andproviding an underfill agent between the chip and the substrate,comprises the steps of applying the underfill agent onto at least one ofthe substrate and the chip, moving the chip to the substrate to bringthe bump into contact with the electrode and to expand the underfillagent, in a space between the chip and the substrate, to around the bumpand the electrode in contact with each other, and heating the bump orelectrode in a state that the bump is buried in the underfill agent tomelt the bump or electrode so as to weld the bump to the electrode.Although the material of the bump or the electrode is not particularlyrestricted, the present invention is suitable for mounting especially ina case where it is formed from a solder.

In the present invention, the bump is formed on at least one of the chipand the substrate. Namely, there are (A) a case where the bump is formedon the chip and the electrode is formed on the substrate, (B) a casewhere the bump is formed on the substrate and the electrode is formed onthe chip, and (C) a case where the bump is formed on each of the chipand the substrate. Therefore, in the case of (C), the “electrode”according to the present invention means a technical concept including abump.

In the chip mounting method according to the present invention, it ispreferred that the underfill agent is expanded up to a side portion ofthe chip by further pushing the chip toward the substrate together withheating the bump or electrode or after the heating. Particularly, it ispreferred that a fillet is formed at the side portion of the chip by theunderfill agent expanded up to the side portion of the chip. By such anunderfill agent expanded sufficiently broad, the bonding strengthbetween the chip and the substrate can be increased as well as a desiredexcellent electrical insulation can be ensured.

Further, it is preferred that, before the chip is mounted to thesubstrate, at least one of the bump and electrode is treated forpreventing a primary oxidation thereof, that may be generated on atleast one of the bump and electrode by the time of the mounting by theatmosphere during storage and the like, or that may be generatedassociated with a certain pre-treatment such as heating treatment forforming a solder bump and the like. As the primary oxidation preventingtreatment, for example, a treatment can be employed, wherein the bump orthe electrode is cleaned by energy wave or energy particles. As theenergy wave or energy particles, for example, any of plasma, ion beam,atomic beam, radical beam and laser can be used. Further, as the primaryoxidation preventing treatment, for example, treatments can also beemployed, wherein a substituent for removing oxygen such as a fluorogroup is chemically bonded to the surface of the bump or electrode,thereby forming a anti-oxidation surface layer, wherein an oxide isremoved from the surface by reducing operation due to hydrogen, andwherein gold is plated to the surface.

In the above-described chip mounting method according to the presentinvention, the underfill agent is applied before chip mounting, andbefore heating, the applied underfill agent is expanded so as to burytherein the bump and the electrode of the substrate in contact with eachother, and therefore, when the bump is heated and molten, the bump iscut off from the surrounding atmosphere by the underfill agent, and thesecondary oxidation of the bump can be prevented. Therefore, the purgingdue to nitrogen gas and the like for preventing the secondary oxidationsuch as a conventional method becomes unnecessary, and the apparatus andthe process can be greatly simplified.

Further, if the aforementioned primary oxidation preventing treatment iscarried out, the application of flux becomes unnecessary, and a furthersimplification becomes possible. Because the cleaning of flux after bumpbonding also becomes unnecessary by making the application of the fluxunnecessary, the process can be simplified also from this point of view.

This chip mounting method according to the present invention is not amethod for injecting an underfill agent into a gap between a chip and asubstrate after preliminarily bonding a bump and an electrode andthereafter mainly bonding them by heating as in a conventional method,but a method for expanding an underfill agent to around a bump of chipand an electrode of a substrate in contact with each other, andperforming a main bonding at a time without performing a preliminarybonding by heating the bump or the electrode in the expanded underfillagent. Because this bonding is carried out through heating and meltingof the bump or the electrode in the underfill agent which does not comeinto contact with oxygen, as mentioned above, there is no fear that thesecondary oxidation occurs by heating. Besides, during this heating andmelting, re-flow necessary for the bump or the electrode is carried outat the same time. Therefore, in the method according to the presentinvention, the mounting is completed substantially by two processes ofthe primary oxidation preventing treatment process and the heating andbonding process of the chip and the substrate including the applicationof the underfill agent, thereby greatly reducing the number of theprocesses for chip mounting and greatly simplifying both the process andthe apparatus.

Furthermore, if the underfill agent is expanded up to the side portionof the chip by further pushing the chip toward the substrate after thebump or the electrode is softened or molten by heating, the bondingstrength and the retaining strength of the chip relative to thesubstrate can be increased. Especially, if the underfill agent isexpanded so as that a fillet can be formed on the side portion of thechip, by forming the fillet, invasion of moisture and the like into aportion between the chip and the substrate can be prevented by theunderfill agent forming the fillet in addition to the underfill agentpresent between the chip and the substrate, and at the same time, itbecomes possible to increase the retaining strength and the bondingstrength of the chip. Further, because a fear that the underfill agentremains between the bump and the electrode can be removed by theabove-described pushing, a more stable electric connection can beachieved therebetween.

Thus, in the chip mounting method according to the present invention, asecondary oxidation of the bump bonding portion can be prevented withoutusing nitrogen gas and the like as in a conventional method, both theapparatus and the process can be greatly simplified, and a chip mountinghaving a high reliability in bonding and insulation properties can becarried out inexpensively.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a schematic side view showing a state before chip mounting ina chip mounting method according to an embodiment of the presentinvention.

FIG. 2 is a schematic vertical sectional view showing a state wherebumps of a chip and electrodes of a substrate are brought into contactwith each other in the chip mounting method shown in FIG. 1.

FIG. 3 is an enlarged sectional view of a bump-electrode portion at atime immediately before reaching the state shown in FIG. 2.

FIG. 4 is a schematic vertical sectional view showing a state where thechip is further pushed from the state shown in FIG. 2.

THE BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, desirable embodiments of the present invention will beexplained referring to figures.

FIG. 1 shows a state immediately before chip mounting in a chip mountingmethod according to an embodiment of the present invention, and shows acase where bumps are formed on the chip side and electrodes are formedon the substrate side. In FIG. 1, numeral 1 shows a chip (for example,an IC chip), and at positions corresponding to the positions ofelectrodes (not shown) provided on the lower surface of the chip, bumps2 (in this embodiment, solder bumps) are formed. Chip 1 is held bysuction on the lower surface of a heat tool 3 of a chip bonding machine.In this embodiment, a primary oxidation preventing treatment such as theaforementioned treatment is performed to the surfaces of at least one ofbumps 2 and electrodes of a substrate, and the solder bonding can becarried out without applying a flux.

As the primary oxidation preventing treatment for bumps 2 or theelectrodes, the aforementioned treatments can be employed such as atreatment for cleaning by energy wave or energy particles, a chemicaltreatment for chemically bonding a substituent such as fluoro groups,and a treatment for plating gold onto the surface of the bump or theelectrode.

Under chip 1, a substrate 5 (for example, a circuit board or a liquidcrystal board) is held on and fixed to a stage for substrate 4 of thechip bonding machine. Electrodes 6 are formed on substrate 5 by, forexample, gold plating. In this bonding machine, the position ofsubstrate 5 held and fixed on the stage 4 is controlled relative to chip1 held by suction on the lower surface of heat tool 3, and particularly,corresponding bumps 2 and electrodes 6 of substrate 5 are aligned toeach other.

Before mounting chip 1 onto substrate 5, underfill agent 7 is applied toat least one of chip and substrate 5, in this embodiment, onto the uppersurface of substrate 5. Although an underfill agent composed of anonconductive adhesive is used as the underfill agent 7 in thisembodiment, as the underfill agent in the present invention, anonconductive paste, a nonconductive film, and an anisotropic conductivepaste and film containing conductive particles such as gold platedplastic particles or solder particles are included. Although it ispreferred that underfill agent 7 is applied after substrate 5 is held onstage 4 from the viewpoint of a desired accurate application inposition, it is possible to apply it before being held. Further, in thestate shown in FIG. 1, underfill agent 7 is applied on substrate 5 at aposition between electrodes 6 so as not to cover each electrode 6 and soas to protrude in a form of a convex. Such a convex style applicationcan be carried out by using a dispenser. Especially, in a case wherechip 1 is further pushed after bumps 2 are heated and molten asdescribed later, underfill agent 7 may be applied even on electrodes 6.As another method, it is possible to apply underfill agent 7 by screenprinting, for example, in a plate-like form partially applied. However,in order to prevent voids from being involved more surely when underfillagent 7 is expanded as described later, it is preferred to apply theunderfill agent 7 in a convex form as described above.

From the state shown in FIG. 1, chip 1 and substrate 5 are approached toeach other. In this embodiment, heat tool 3 is moved downward, and bumps2 of chip 1 are brought into contact with corresponding electrodes 6 ofsubstrate 5. At that time, as shown in FIG. 2, underfill agent 7 havingbeen applied on substrate 5 is pushed and expanded between chip 1 andsubstrate 5, and it fills a gap therebetween. Further, at that time,although the expanded underfill agent 7 is likely to be interposedbetween bumps 2 and electrodes 6, or, underfill agent 7 is likely to bepresent on electrodes 6 from the stage that the underfill agent 7 isapplied prior to the mounting, the underfill agent 7 having been presentbetween bumps 2 and electrodes 6 is pushed away toward around them inthe process in which bumps 2 come into contact with electrodes 6, asshown in FIG. 3. Particularly, by moving down heat tool 3 at anappropriate pressing force and bringing bumps 2 into contact withelectrodes 6 at an appropriate pressure, at least a part of the tipportion of each bump 2 can be brought into contact directly with eachelectrode 6 without interposing underfill agent 7. Further, because thetip portion (tip portion of the lower surface) of bump 2 is formed as ashape close to a spherical surface, underfill agent present around thetip portion is pushed away to around the bump 2 and the electrode 6 incontact with each other efficiently and smoothly.

As shown in FIG. 2, bumps 2 and electrodes 6 in contact with each otherare in a condition where they are buried in expanded underfill agent 7,and they are substantially intercepted from the surrounding atmospherecompletely. At that time, by pressing chip 1 at an adequate pushingforce, or by controlling the position (height) of chip 1 adequately, andfurther, by controlling the temperature of underfill agent 7 adequately,a preferable pressing state between bumps 2 and electrodes 6 and apreferable expansion state of the underfill agent 7 can be bothachieved. In this state, chip 1, ultimately, solder bumps 2, are heatedat a predetermined temperature by heat tool 3, and the solder bumps 2are molten and bonded to electrodes 6. Because bumps 2 are completelyisolated from the surrounding atmosphere, a secondary oxidation does notoccur on the bumps 2 being heated and molten. Therefore, purging of theatmosphere by nitrogen gas and the like for preventing the secondaryoxidation as in a conventional method is unnecessary, thereby greatlysimplifying both the apparatus and the process.

Further, in this embodiment, since a primary oxidation preventingtreatment is carried out for bumps 2 in advance, application of fluxsuch as in a conventional method is unnecessary, and cleaning of fluxafter heating and bonding, which is required in a case of fluxapplication, is also unnecessary. Therefore, the process can be furthersimplified.

Furthermore, bumps 2 are heated at a predetermined temperature andmolten in underfill agent 7, the bumps 2 can sufficiently wet-expand ina state prevented from secondary oxidation, and therefore, ahigh-reliability bonding between bumps 2 and electrodes 6 can beachieved. Because a desired bonding between bumps 2 and electrodes 6 canbe achieved at a state prevented from secondary oxidation, it becomespossible to perform a main bonding at a time without separating the mainbonding from a preliminary bonding, and the number of processes requiredby the time reaching the completion of the main bonding can be greatlydecreased. From this point of view, both the apparatus and the processcan be greatly simplified.

Although the apparatus and the process can be greatly simplified only bythe above-described stages, further in this embodiment, a furtherpressing process of chip 1 is added as shown in FIG. 4. Although it ispossible to omit this further pressing process of chip 1, addition ofthis process is preferable from the viewpoint of further increasing thebonding strength and the electrical insulation property.

As shown in FIG. 4, from the time when bumps 2 are heated and the bumps2 are at least softened, or from the time when the bumps 2 are heatedand molten, heat tool 3 is further moved down slightly, and chip 1 isfurther pressed toward substrate 5. By this action, underfill agent 7having been expanded between chip 1 and substrate 5 is further pressedand expanded up to the side portion of chip 1. In this embodiment,particularly, the underfill agent 7 is pressed and expanded so as that afillet 8 due to the underfill agent 7 is formed on the side portion ofchip 1. Therefore, the above-described additional pushing amount may bean amount corresponding to that capable of forming fillet 8. By such afurther pressed and expanded underfill agent 7, the bonding strengthbetween chip 1 and substrate 5 can be increased. Especially, by formingfillet 8, as compared with a case of no fillet, chip 1 is bonded to andheld by substrate 5 more strongly. Further, because the bonding portionsof bumps 2 and electrodes 6 are sealed from the surrounding environmentmore completely, the reliability in electrical insulation can be furtherincreased. Furthermore, because the bonding area of bumps 2 is increasedby the further pressing, the reliability in electrical insulation andthe bonding strength can be further increased. Where, since there may bea case where bumps 2 are broken by the pressing if only a simplepressure control is carried out, it is preferred to control the pressingheight, for example, by detecting the further pressing amount from thecondition, where bumps 2 and electrodes 6 come into contact with eachother, by a sensor. Further, it is possible to correct the shapes ofbumps 2 by slightly lifting chip 1 after the pressing.

In this state, underfill agent 7 is cured. Then, cooling is carried out,and the bonding portions of bumps 2 and electrodes 6 are solidified.Complete cure of underfill agent 7 may be late in time as compared withthe solidification of the bonding portions.

Although bumps 2 of the side of chip 1 are formed as solder bumps in theabove-described embodiment, it is possible to form electrodes 6 of theside of substrate 5 as a solder constitution, and it is also possible toform both of them as a solder constitution. Further, in the presentinvention, the formation of chip 1 is not particularly limited, and aslong as having a bump to be metal bonded in which a secondary oxidationdue to heating becomes a problem, the present invention can beeffectively applied to any type of chip. Further, the formation ofsubstrate also is not particularly limited, and the present inventioncan be applied to any type of substrate to which a chip having a bump ismounted. Furthermore, although an aspect forming bumps 2 on the side ofchip 1 and forming electrodes 6 on the side of substrate 5 is explainedin the above-described embodiment, in the present invention, the bump isnot limited to the chip side, and the bump may be formed on thesubstrate side or on both the chip side and the substrate side. In acase where bumps are formed on both sides, as aforementioned, an“electrode” in the present invention means a technical concept includinga bump.

In particular, the present invention is suitable for bonding of objectshaving many bumps or for bonding of objects formed with bumps at a finepitch. Particularly, in a case of many bumps, the present invention canbe easily carried out by setting a pressure per one bump adequately.Further, even in a case of an object having bumps at a fine pitch, it isnot necessary to inject an underfill agent into a gap between a chip anda substrate approached to each other, and the present invention can beeasily carried out because the underfill agent may be applied initially.

Further, as the primary oxidation preventing treatment for a bump or anelectrode, as aforementioned, treatments can be employed such as atreatment for cleaning by energy wave or energy particles, a chemicaltreatment for chemically bonding a substituent such as fluoro groups,and a treatment for plating gold onto the surface of the bump.Furthermore, a method for cleaning by using energy wave or energyparticles or by using a solution, and thereafter, mounting the cleanedobject under a condition controlling a time (in a predetermined time)before the object is oxidized again, or a method for mounting the objectin an Ar or N₂ atmosphere after the above-described cleaning, can alsobe employed.

INDUSTRIAL APPLICATIONS OF THE INVENTION

In the chip mounting method according to the present invention, becausethe apparatus and the process can be both greatly simplified, and thesecondary oxidation of bumps can be prevented and the mounting can becarried out with a high reliability in insulation property, the presentinvention can be applied to any field requiring the mounting of a chipto a substrate. Particularly, the present invention is suitable to aprocess for mounting many chips in a short period of time, and cancontribute to a great simplification of the process, a great reductionof the cost of the apparatus, a great shortening of the tact time, and agreat increase in quality.

What is claimed is:
 1. A method for mounting a chip by bonding a bumpformed on at least one of the chip and a substrate to an electrodeformed on the other and providing an underfill agent between the chipand the substrate, comprising the steps of: applying the underfill agentonto at least one of the substrate and the chip spaced apart from thebump or the electrode so as to allow the electrode to remain uncoveredand so as to protrude in a form of a convex surface; moving the chip tothe substrate to bring the bump into contact with the electrode and todistribute the underfill agent, in a space between the chip and thesubstrate, to around the bump and the electrode in contact with eachother; after completing the distribution of the underfill agent, heatingthe bump or the electrode in a state that the bump or the electrode iscompletely buried in the underfill agent to melt at least one of thebump and the electrode so as to weld the bump to the electrode; andfurther pressing the chip toward the substrate during or after said stepof heating the bump or the electrode so that the underfill agent isdistributed beyond a side portion of the chip so as to form a filletwhich covers a portion of a side surface of the chip.
 2. The chipmounting method according to claim 1, wherein, before the chip ismounted to the substrate, said method further comprising the step oftreating at least one of the bump and the electrode for preventing aprimary oxidation thereof that may be generated by the time of themounting.
 3. The chip mounting method according to claim 2, wherein saidstep of treating includes cleaning at least one of the bump and theelectrode by an energy wave or energy particles for preventing theprimary oxidation.
 4. The chip mounting method according to claim 3,wherein said step of cleaning includes using a plasma, ion beam, atomicbeam, radical beam or laser as the energy wave or energy particles. 5.The chip mounting method according to claim 2, wherein said step oftreating includes chemically bonding a substituent for removing oxygento a surface of at least one of the bump and the electrode, or, using areducing operation including hydrogen to remove an oxide from thesurface for preventing the primary oxidation.
 6. The chip mountingmethod according to claim 2, wherein said step of treating includes goldplating at least one of the bump and the electrode for preventing theprimary oxidation.
 7. The chip mounting method according to claim 1,wherein said step of further pressing the chip includes forming a filletwhich covers an entire side surface of the chip.
 8. A method formounting a chip by bonding a bump formed on a chip to an electrodeformed on the substrate, comprising the steps of: applying the underfillagent onto the substrate spaced apart from the electrode so as to allowthe electrode to remain uncovered and so as to protrude in a form of aconvex surface; moving the chip to the substrate to bring the bump intocontact with the electrode and to distribute the underfill agent, in aspace between the chip and the substrate to bury the bump and theelectrode in the underfill agent; heating the bump or the electrode tomelt at least one of the bump and electrode so as to weld the bump tothe electrode; and further pressing the chip during or after heating thebump or the electrode so that the underfill agent is distributed beyonda side of the chip so as to form a fillet that covers a portion of aside surface of the chip.
 9. The chip mounting method according to claim8, wherein said step of further pressing the chip includes forming afillet which covers an entire side surface of the chip.
 10. A method formounting a chip by bonding a bump formed on at least one of the chip anda substrate to an electrode formed on the other and providing anunderfill agent between the chip and the substrate, comprising the stepsof: applying the underfill agent onto at least one of the substrate andthe chip spaced apart from the bump or the electrode so as to allow theelectrode to remain uncovered and so as to protrude in a form of aconvex surface; moving the chip to the substrate to bring the bump intocontact with the electrode and to distribute the underfill agent, in aspace between the chip and the substrate, to around the bump and theelectrode in contact with each other; after completing the distributionof the underfill agent, heating the bump or the electrode in a statethat the bump or the electrode is completely buried in the underfillagent to melt at least one of the bump and the electrode so as to weldthe bump to the electrode; and further pressing the chip toward thesubstrate during or after said step of heating the bump or the electrodeso that the underfill agent is distributed beyond a side portion of thechip so as to form a fillet which completely covers a side surface ofthe chip.